As for the second point, I think it is not bad to tell someone to invest in bitcoin if the time before the person will retire is still far. And before then as I said, tell the person both the pros and cons so that you not be held responsible when there is any downfall. And it is very wrong for someone to start planning to invest when he about to stop working. There is no need for that unless the person has another business running to fetch him coolly money.
I don't also think that it is bad to introduce people to invest in Bitcoin. Investing in Bitcoin is not illegal and it can be profitable. After all, it was someone who introduced some of us to Bitcoin. Some people might blame you in future for not advising them to invest in it. We shouldn't only look at the negative consequences, the investment might become fruitful.
But also understand the reservations of Mpamaegbu, some retirees don't have other sources of income and depend solely on the pension or gratuity. I will suggest that your pastor invest part of his retirement benefits and keep others for his upkeep. But if he has other sources of income that can sustain him, Bitcoin is a good investment. But don't forget to teach him how to keep his funds safe.